FIELD OF THE INVENTION
The present invention relates to a process for preparing fatty acid esters and/or mixtures of fatty acid esters of short-chained monohydric alcohols or monoalkylated diols by trans-esterification of fatty acid glycerides with the short-chained alcohols or monoalkylated diols in the presence of basic catalysts.
The fatty acid esters prepared according to the invention are suitable, depending on the starting materials used, as pharmaceutical, dietetic or cosmetic raw materials, as intermediate products for further fatty acid derivatives, such as fatty alcohols, fatty amines, surfactants and so forth, and also as lubricants, plasticizers, hydraulic oils, fuels, and Diesel fuels.
Because of their suitability as Diesel fuel, such fatty acid esters have recently gained remarkable significance, for reasons of environmental protection, replacement of fossil fuels with renewable energy sources, and in connection with problems that arise in agriculture.
The preparation of fatty acid esters by basically catalyzed trans-esterification has long been known. A survey of known processes may be found in J.A.O.C. Soc. 61 (1984), page 343ff., and in Ullmann, Enzyklopadie d. techno Chemie [Ullmann's Encyclopedia of Chemical Engineering], Fourth Edition, Vol. 11, page 432. In all these processes, the preparation is done by mixing fatty acid glycerides, and in particular animal or vegetable oils and fats, with an excess of the short-chain alcohols, with the addition of a basic catalyst and separation of the subsequently forming heavier glycerine phase, under reaction conditions that very depending on the quality of the starting materials.
The substantial disadvantages of the known processes are that the fatty acid glycerides, if the trans-esterification is to be done at temperatures below 100.degree. C. and without pressure, have to be cleaned after their recovery, for instance by pressing or extraction, and in particular must be freed of free fatty acids and phosphatides. If by comparison uncleaned fatty acid glycerides are used, then high pressures and high temperatures, or other provisions such as pre-esterification of the free fatty acids, alcohol vapor countercurrent processes, etc., must be employed. All these provisions require major engineering and equipment expense and hence high investment costs and make it difficult or impossible to employ these processes for recovering renewable and environmentally friendly energy sources in small systems, for instance in the context of agricultural operations or in developing countries.
Other disadvantages or difficulties in the known processes arise in the phase separation between the lighter fatty acid ester phase and the heavier glycerine phase, which is done either very slowly or, because of emulsification, incompletely or not at all; this applies above all to raw, hot-pressed or extracted vegetables oils with a high phosphatide content.
Separating the components of the basic catalyst that remain in the fatty acid ester phase presents another difficulty. If this separation is done by washing with water or with dilute acids, then emulsification, especially if phosphatide-rich oils are used, and the major production of waste water present problems. If ion exchangers are used, then their regeneration and the waste water produced also present problems.
The need therefore exists for a process that at minimal engineering and equipment expense furnishes fatty acid esters of high purity, as required for instance for use as Diesel fuel, by using raw uncleaned vegetable or animal oils and fats, in particular those with a high content of free fatty acids and phosphatides, with fast, complete separation of the glycerine phase and with simple removal of the catalyst residues.